Total-internal-reflection is the working principle of the light transmission in a fiber in which a few amount of the electromagnetic field is penetrated outside the vicinity of the guiding core with exponentially decayed field distribution, called an evanescent field. For a side-polished fiber, the evanescent field of the guided light can be accessed since a portion of the cladding is removed. The guided light can interact with an external medium through evanescent coupling and will be well-confined to propagate forward as long as the total-internal-reflection criterion is satisfied. Thus, the refractive index of the medium placed against the polished surface should be smaller than the effective refractive index of the side-polished fiber.
As everyone knows, the refractive index of the fused silica fiber is around 1.45 for 1.55 μm wavelength. In contrast to most of the physical substances, the refractive index of the fused silica is too low for the side-polished fiber to find a suitable medium to achieve a tunable wavelength filter, an intensity modulator or an optical amplifier. As a result, to develop variant kinds of the fiber components for fiber-optic communications is seriously restricted.
A tunable wavelength filter plays an important role in bandwidth provisioning and management for the photonic networks. It transmits the required wavelength(s) and rejects the rest in front of the receiver. Needless to say, a tunable filter is convenient and cost-effective since pluralities of the fixed filters are replaced.
Presently, several kinds of fabrication methods are demonstrated to make tunable filters, such as photo-induced or acoustic-optic fiber grating, Fabry-Perot resonantor, thin-film or Mach-Zehnder interferometer. However, the above methods are highly related to a stable mechanical system whose precision and reliability are questionable.
Particularly, the tuning range of the fiber Bragg grating is normally less than 30 nm through a repeated pulling or compressing, which could shorten the lifetime of the grating.
As to the Fabry-Perot filter, it is wideband tunable. However, the critical requirements on mechanical stability and the demand of an expensive controller have made this filter difficult to be widespread and popular.
The tunable thin-film filter is not suitable for a high density wavelength-division-multiplexing communication system because the working principle of this filter is to rotate the filter until the required angle and the corresponding wavelength are selected. The amounts of coating layers of this filter for high density system is more than 200 at least and therefore the temperature control is extremely important to keep the stable interferences.
The acousto-optic tunable filter exhibits a tuning range of wider than 100 nm wavelength based on photo-elastic effect. The propagating acoustic wave induces a periodic index grating that diffracts certain guided wavelength. However, the exciting efficiency and propagating distance of the acoustic wave are the concerned issues. The accuracy of the induced grating period would also be a challenge. Moreover, the high frequency signal generator for exciting the acoustic wave is an extremely expensive instrument. Therefore, the acousto-optic tunable filter is difficult to be popularized.
Many efforts have been done to solve the above problems in the following studies:
A method of employing a fan-shaped grating in the evanescent-field area of the side-polished fiber and using a precision micro-meter to move the grating for adjusting the grating period was disclosed by Sorin with the Stanford University in 1985. A tunable fiber filter with a wavelength tuning range of 65 nm, a reflectivity of 88% and a FWHM (Full Width at Half Maximum) of 1 nm is manufactured by this method. However, the tuning through a mechanical method damages the fiber probably and it would be a question to guarantee the accuracy and reproduce the initial status. Hence this method is impractical.
X.-Z. Lin et al. with the Beijing University (1994) utilized a magnetic material for being as a substrate of the polished fiber and then fabricated a surface-relief grating on the polished fiber. When a magnetic field of high intensity is applied to the substrate, the grating period is compressed accordingly and thus an electrical-controlled tunable filter is achieved. However, the method is not practical because only a wavelength span of 1.3 nm is achieved while applying a high intensity magnetic field of up to 0.1 T.
A method of employing the piezoelectric effect to generate a force to pull and compress the grating by applying a high voltage to the fiber grating placed on a piezoelectric material, was disclosed by A. Iocco et al. with the Swiss Federal Institute of Technology in 1997. The grating period is adjusted by the tension and the compression and a wavelength tuning range of 15˜30 nm is obtained by this method. However, the piezoelectric material needs a high voltage to drive and therefore lacks of the practicability.
Katsumi Takano et al. with the Yamagata University in Japan (2001) disclosed a wavelength tuning method which utilizes a MEMS (Micro-Electro Mechanical Systems) technique to rotate the angle of the defect in a photonic crystal so as to change the wavelength. However, this method is extremely complicated and difficult to be implemented.
In order to overcome the foresaid drawbacks in the prior art, a fiber-optic tunable filter and an intensity modulator are provided in the present invention.